Hydrostatic hybrid drive system

ABSTRACT

The invention relates to an hydrostatic hybrid drive system for road vehicles, said system comprising a pump/motor unit ( 1 ) which is or can be connected to the powertrain of the vehicle, can be controlled by means of a control unit ( 5 ) in a pump or motor operation and can be connected to a high-pressure hydraulic accumulator via a first working line ( 7 ) that can be closed by means of a control valve ( 45 ) and to a low-pressure hydraulic accumulator via a second working line ( 9 ). The high-pressure hydraulic accumulator and low-pressure hydraulic accumulator are formed by a double-piston accumulator ( 11 ), in which a high-pressure side and a low-pressure side having respective accumulator pistons ( 15, 17 ) are formed in an accumulator housing ( 13 ), wherein the fluid chambers ( 23  and  25 ) of the high-pressure side and the low-pressure side are separated by a central housing part ( 21 ) through which the common piston rod for both accumulator pistons ( 15, 17 )extends. The invention is characterised in that the control unit ( 5 ) of the pump/motor unit ( 1 ) having a control area can be supplied from the first working line ( 7 ) via a line connection ( 59 ) which is provided on the first working line ( 7 ) between the pump/motor unit ( 1 ) and the control valve ( 45 ).

The invention relates to a hydrostatic hybrid drive system that isintended for road vehicles and that comprises a pump/motor unit, whichis or can be connected to the drive train of the vehicle, can becontrolled by means of a control unit in a pump or motor mode, and canbe connected to a high pressure hydraulic accumulator by means of afirst working line, which can be closed by means of a control valve, andcan be connected to a low pressure hydraulic accumulator by means of asecond working line, with the high pressure hydraulic accumulator andthe low pressure hydraulic accumulator being formed by a double pistonaccumulator having a high pressure side and a low pressure side, each ofwhich has an accumulator piston and is formed in an accumulator housing,so that the fluid chambers of the high pressure side and the lowpressure side are separated by a central housing part through which thecommon piston rod for both accumulator pistons extends.

In light of the scarcity of natural resources and the increasing impactof CO₂ on the environment, the current trend in automotive engineeringis to use hybrid drive systems. The systems that are currently in useusually involve electric motor powered hybrids, which store the electricenergy generated in braking modes and recover drive energy from thestored energy in order to provide assistance to the vehicle for thedrive mode and, in particular, for acceleration processes. This strategyoffers the possibility of decreasing the drive power of the internalcombustion engine, which serves as the primary drive, for comparableroad performance. The result of such a “downsizing” is not only areduction in fuel consumption, but it also raises the possibility ofassigning the vehicles concerned to a lower emissions class thatsatisfies a lower-cost road tax category.

These goals can also be achieved with a hydraulic hybrid system owing tothe high energy density and the compact design of hydraulic systems. Inorder to make available additional drive torque even at low speeds andstarting from zero speed for acceleration processes or in order toprovide assistance to the braking action during braking modes, such ahydraulic hybrid system stores the hydraulic energy in a hydraulicaccumulator by means of a pump/motor unit so that this hydraulic energycan be used, as required, as the drive energy when the pump/motor unitis in motor mode. Such a hydrostatic drive system with recovery of thebraking energy is disclosed in document DE 601 18 987 T2.

Based on the aforementioned prior art, the object of the presentinvention is to provide a hydrostatic hybrid drive system that isdistinguished by especially good operating behavior for the intendedpurpose.

This object is achieved with a drive system having the featuresspecified in claim 1 in its entirety.

According to the characterizing part of claim 1, an essential feature ofthe invention resides in the fact that the control unit of thepump/motor unit can be supplied with control fluid from the firstworking line by way of a line connection, which is provided on the firstworking line between the pump/motor unit and the control valve. Thisfeature guarantees that the control unit will be supplied in anoperationally reliable way with the control pressure, required for afast reaction of the control functions of the control unit,independently of the respective operating mode.

In this context, an especially advantageous embodiment can provide thata non-return valve is disposed between the first and the second workingline; that this non-return valve can be opened by pressure actuation inthe direction of the first working line; that the leakage fluid of thepump/motor unit can be delivered to the second working line; and thatthe leakage fluid can be delivered to the second working line by way ofa charging pump. This feature can not only compensate for the leakage bypumping to the second working line, but it also allows the pressurelevel in the line, which is protected by means of a pressure limitingvalve, to be adjusted to a pressure level that is raised by such asuitable amount that it ensures that the control unit is supplied with acontrol pressure of a suitable pressure level from the first workingline by way of the non-return valve disposed between the second workingline and the first working line, even in the case of operating states inwhich the control valve is closed.

In advantageous embodiments, the control unit of the pump/motor unit canbe supplied with filtered control fluid by way of a pressure reducingvalve that has an upstream fluid filter and is connected to the firstworking line.

Preferably, the pump/motor unit is formed by an axial piston machine,which can be reversed between pump mode and motor mode by changing thepivot angle beyond the zero angle.

In order to reduce the pressure differential of the high pressure sideof the double piston accumulator between the charged and the dischargedstate, an N₂ supply tank is connected preferably to the gas side of thehigh pressure side of the double piston accumulator.

Since the system according to the invention has a low pressure levelthat is constant owing to the use of the double piston accumulator, thispressure can be adjusted by means of a small auxiliary accumulator thatis connected to the second working line and that also compensates forcompression losses.

The object of the invention is also a double piston accumulator that isprovided, in particular, for use in a drive system according to one ofclaims 1 to 7 and that exhibits the features disclosed in claim 8.

The invention is explained in detail below by means of embodiments thatare depicted in the drawings.

Referring to the drawings:

FIG. 1 is a highly simplified symbolic representation of just thehydraulic circuit of an embodiment of the inventive drive system withoutthe peripheral components, such as the control units and thevehicle-side mechanical components.

FIG. 2 is a diagrammatic representation of the curve of the accumulatorpressure levels.

FIGS. 3 and 4 are a top view and a front view respectively of a doublepiston accumulator of the system according to the invention; and

FIG. 5 is a sectional view along an intersecting line VI-VI from FIG. 4.

FIG. 1 shows the hydraulic circuit of an embodiment, where a pump/motorunit is designated as 1. This pump/motor unit is driven in a constantdirection of rotation by an internal combustion engine of the vehicleconcerned. This internal combustion engine, like the rest of the partsof the vehicle, is not depicted. In the present example, the pump/motorunit 1 is driven directly by the crankshaft 3 (depicted in schematicform) of the internal combustion engine. The pump/motor unit 1 is anaxial piston machine having a pivot angle that can be adjusted beyond aneutral zero position in both pivotal directions by means of anelectro-hydraulic control unit 5 so that the pump/motor unit 1 can workin the pump mode and in the motor mode at a constant direction ofrotation. The control unit 5 interacts with the vehicle's electronicengine management, which conforms to the prior art, by way of aninterface that is not depicted.

A first working line 7 is connected to the high pressure side of thepump/motor unit 1; and a second working line 9 is connected to the lowpressure side of the pump/motor unit 1. The first working line 7, as thehigh pressure line, and the second working line 9, as the low pressureline, form together with a double piston accumulator 11 a kind ofhydraulic cradle, in which the pressure fluid can be conveyed to thedouble piston accumulator 11 and back from the double piston accumulator11. The double piston accumulator 11 fulfills the functions of a highpressure accumulator and a low pressure accumulator. For this purpose,there is a common accumulator housing 13 in which a high pressureaccumulator piston 15 and a low pressure accumulator piston 17 can bemoved, and both the high pressure accumulator piston and the lowpressure accumulator piston are connected rigidly together by means of acommon piston rod 19. The piston rod 19 extends through a centralhousing part 21, which separates a fluid chamber 23 of the high pressureside from a fluid chamber 25 of the low pressure side. In order to fillthe gas side 27, which abuts the high pressure accumulator piston 15,with working gas, a supply tank 19 with N₂ gas is connected to the gasside 27.

In order to supply the control unit 5 with control fluid, a supply line31 is connected to the first working line 7 in order to deliver filteredcontrol fluid to a supply port 37 by way of a fluid filter 33 and apressure limiting valve 35. At the same time, the control pressure levelcan be adjusted with respect to the first working line 7 and/or thesecond working line 9 by means of the pressure limiting valves 39 and41. Between these working lines, there is a non-return valve 43, whichcan be opened by pressure actuation in the direction of the firstworking line 7. In order to prevent the pump/motor unit 1 from leakingwhile the system is under no load at the pivot angle zero, the firstworking line 7 can be closed by a control valve 45.

When a drive system of the conventional type, in which a first workingline and a second working line are connected to separate hydraulicaccumulators, which contain their own working gas volumes, is running,the pressure level of the low pressure accumulator drops as the pressurelevel of the high pressure accumulator rises, so that it is difficult toreplenish the system with leakage fluid of the pump. However, in thecase of the double piston accumulator 11 provided according to theinvention, the sum of the fluid volumes in the high pressure fluidchamber 23 and the low pressure fluid chamber 25 is always constant forall piston positions, so that, owing to the jointly movable accumulatorpistons 15 and 17, the pressure level in the second working line 9—thatis, the low pressure line—remains constant. FIG. 2 shows the respectivepressure curves for the stored energy in separate hydraulic accumulatorsand for the double piston accumulator 11 provided according to theinvention.

When the pressure level in the second working line 9 is constant, thepressure level can be adjusted to an optimal value by means of anattached, small auxiliary hydraulic accumulator 47, which alsocompensates for compression losses.

In the embodiment from FIG. 1, the leakage fluid of the pump 1 isdelivered to the second working line 9 by way of a filter 49 with abypass 51 and by way of a charging pump 55. Since the housing pressureof the pump 1 corresponds to the pressure level of the second workingline 9, this second working line is protected by a pressure limitingvalve 53 for safety reasons.

When an axial piston machine for closed loop operation is used as thepump/motor unit 1, the resulting higher flow losses make it necessary towork with a higher low pressure level. Since, however, a higher pressurelevel is not admissible as the housing pressure of the pump, anadditional leakage line has to be provided in order to protect thehousing gasket from an overload. Pumps for closed loop operation alsorequire a pressure differential between the housing and the low pressureside in order to hold the piston at the swivel plate. In view of thisrequirement, the embodiment from FIG. 1 has a small, electric motordriven charging pump 55 that is arranged in a separate line 57 in orderto compensate for the leakage by pumping in the direction of the secondworking line 9. At this point, the housing pressure is uncoupled fromthe low pressure level.

At this point, the higher low pressure level in the second working line9 offers the possibility of supplying the control unit 5 with controlfluid from the second working line 9—that is, by way of the non-returnvalve 43. For this purpose, the pressure limiting valve 35 in the secondembodiment is connected to the first working line 7 at 59 between thepump unit 1 and the control valve 45. This first working line candeliver control fluid to the port 37 by way of the opened control valve45, the filter 33, and the pressure limiting valve 35. When thenon-return valve 43 is closed, a higher control pressure is madeavailable now for the control unit 5.

FIGS. 3 to 5 show a practical embodiment of the double pistonaccumulator 11, which has a common accumulator housing 13 for bothaccumulator pistons 15 and 17. Said common accumulator housing has ahigh pressure side housing half 63 and a low pressure side housing half65, both of which are separated from each other by a central housingpart 21. The piston rod 19, which connects together the two pistons 15and 17 in a rigid manner, extends through the central part 21 with afluid-tight seal. A position sensor 69 extends from the open end 67 ofthe low pressure side housing half 65 into an inside borehole 71 of thepiston rod 19 in order to deliver an indication of the piston positionfor the system.

One special feature of the double piston accumulator 11 resides in thefact that the central housing part 21 forms a kind of valve block, atwhich and in which all of the line connections and other components arelocated. In this context, the illustrated example shows the ports 73 and75 for a first working line 7 and a second working line 9 respectively.Furthermore, the central housing part 21 contains the control valve 45,the filter 33, and the pressure limiting valve 35, the control fluidport 37, and the pressure sensors 77 and 79.

An additional advantage over the electric hybrid systems resides in thefact that the system according to the invention can be installed intoexisting vehicles without any difficulty, because the pump/motor unit 1can be mechanically coupled directly to the drive train.

1. A hydrostatic hybrid drive system that is intended for road vehiclesand that comprises a pump/motor unit (1), which is or can be connectedto the drive train of the vehicle, can be controlled by means of acontrol unit (5) in a pump or motor mode, and can be connected to a highpressure hydraulic accumulator by means of a first working line (7),which can be closed by means of a control valve (45), and can beconnected to a low pressure hydraulic accumulator by means of a secondworking line (9), wherein the high pressure hydraulic accumulator andthe low pressure hydraulic accumulator are formed by a double pistonaccumulator (11) having a high pressure side and a low pressure side,each of which has an accumulator piston (15, 17) and is formed in anaccumulator housing (13), so that the fluid chambers (23 and 25) of thehigh pressure side and the low pressure side are separated by a centralhousing part (21) through which the common piston rod for bothaccumulator pistons (15, 17) extends, characterized in that the controlunit (5) of the pump/motor unit (1) can be supplied with control fluidfrom the first working line (7) by way of a line connection (59), whichis provided on the first working line (7) between the pump/motor unit(1) and the control valve (45).
 2. The drive system according to claim1, characterized in that a non-return valve (43) is disposed between thefirst (7) and the second working line (9), that this non-return valvecan be opened by pressure actuation in the direction of the firstworking line (7), that the leakage fluid of the pump/motor unit (1) canbe delivered to the second working line (9), and that the leakage fluidcan be delivered to the second working line (9) by way of a chargingpump (55).
 3. The drive system according to claim 1, characterized inthat the control unit (5) of the pump/motor unit (1) can be suppliedwith filtered control fluid by way of a pressure reducing valve (35)that has an upstream fluid filter (33) and is connected to the firstworking line (7) by means of the line connection (59).
 4. The drivesystem according to claim 1, characterized in that the pump/motor unit(1) is formed by an axial piston machine, which can be reversed betweenpump mode and motor mode by changing the pivot angle beyond the zeroangle.
 5. The drive system according to claim 1, characterized in thatan N₂ supply tank (29) is connected to the gas side (27) of the highpressure side of the double piston accumulator (11).
 6. The drive systemaccording to claim 1, characterized in that an auxiliary hydraulicaccumulator (47) is connected to the second working line (9).
 7. Thedrive system according to claim 1, characterized in that the secondworking line (9) is protected by a pressure limiting valve (53).
 8. Adouble piston accumulator, which is provided, in particular, for use ina drive system, according to one of the preceding claims, wherein a highpressure side and a low pressure side having in each case an accumulatorpiston (15, 17) are formed in a common accumulator housing (13), so thatthe fluid chambers (25, 27) of the high pressure side and the lowpressure side are separated by a central housing part (21), throughwhich the common piston rod (19) for both accumulator pistons (15, 17)extends, and wherein the central housing part (21) is formed by a valveblock, which contains the hydraulic components, such as control valve(45), pressure reducing valve (35), pressure sensors (77, 79), and/orfluid filter (33).